This folder cannot be added. It contains a folder that is already syncing.

This error appears right after a job configuration or when trying to start a job.

It means the following misconfiguration: the Agent is added to two different jobs. In one of the jobs, it's configured to sync a folder with Read-Only permissions, while in the other - its parenting or child folder with Read-Write permissions.

The only fix for it is to revise the folders that the Agent syncs. The best way to see the details is to go to the AGENTS tab -> Agent -> Jobs and review its jobs.